To start communication of packet data by using a mobile communication apparatus, represented by a cellular phone, a packet call, which is a call for packet communication, is established first, then transmission/reception of packet data is started. Specifically, the mobile communication apparatus transmits a PDP (Packet Data Protocol) establishment request message to a mobile communication network, for example, and receives a PDP establishment response message from the mobile communication network in response to this message. By this, the packet call is established. Then the mobile communication apparatus receives the packet data from the mobile communication network. This technology, related to PDP establishment messages, is disclosed in Patent Document 1, for example.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-174671